In a DRAM (Dynamic Random Access Memory), tantalum oxide (TaO) or another metal oxide dielectric substance is preferably used as the material of the capacitive insulating film of a cell capacitor. Described below is a conventional method of forming a cell capacitor for a DRAM in which such a capacitive insulating film is used.
First, a lower electrode made of polycrystalline silicon is formed on the surface of a base structure on which cell transistors have been formed. Hemispherical silicon grains (HSG) have preferably been formed on the surface of the lower electrode in this case (see Japanese Patent Laid-open No. 2004-063964). An oxidation-preventing silicon nitride film (Si3N4) is then formed on the surface of the lower electrode by subjecting the base structure to rapid thermal nitridation (RTN) in an ammonia (NH3) gas atmosphere at about 650° C. or higher, or in a plasma-activated ammonia (NH3) gas atmosphere (see Japanese Patent Laid-open No. 2003-115548).
Non-crystalline tantalum oxide acting as the capacitive insulating film is subsequently deposited by CVD (see Japanese Patent Laid-open No. 2001-036031). Intermediate annealing is then carried out to improve the quality of the non-crystalline tantalum oxide film, after which high-temperature annealing is performed to crystallize the non-crystalline tantalum oxide film. An upper electrode comprising titanium nitride (TiN) is lastly formed on the capacitive insulating film.
In addition to tantalum oxide (TaO), also known as capacitive insulating films for DRAM are aluminum oxide (Al2O3), hafnium oxide (HfO2), and other metal oxide dielectric substances (see Japanese Patent Laid-open No. 2002-094015). These substances have been particularly noteworthy in recent years because of their excellent compatibility with polycrystalline silicon electrode films, which serve as the lower electrode, in comparison with tantalum oxide (TaO).
There is a problem with the above-described conventional manufacture of a cell capacitor for a DRAM in that the frequent heat treatments activate the heavy metals in the silicon substrate, the impurities that affect the destruction of junctions, and other contaminants, and the refresh characteristics are therefore adversely affected by an increase in junction leaks. In order to solve this problem, it is possible to consider a method whereby a slow cooling process is added for the purpose of gettering after the formation of cell capacitors in which tantalum oxide (TaO) is used as a capacitive insulating film. However, this approach is liable to lead not only to a reduction in manufacturing efficiency due to increased labor, but also to a reduction in the quality of the capacitive insulating film comprising tantalum oxide (TaO).
In view of the above, it is possible to focus on the fact that high-temperature annealing must be performed in order to crystallize the film after the non-crystalline oxide tantalum film has been formed in cases in which tantalum oxide (TaO) is used as the capacitive insulating film, and to consider a method whereby contaminants are gettered by carrying out a slow cooling process during the high-temperature annealing step.
When, however, aluminum oxide (Al2O3) and hafnium oxide (HfO2) are used as the capacitive insulating film, high-temperature annealing is not required to crystallize the film as is the case of tantalum oxide (TaO), the heating temperature during film formation is not exceedingly high at about 450° C., and the procedure directly advances to the upper electrode formation step after the capacitive insulating film has been formed. Therefore, a problem is encountered in that there is no opportunity to carry out a slow cooling process after the capacitive insulating film has been formed.